Motion artifacts typically are a serious issue in perfusion imaging, in particular for dynamic contrast-enhanced (DCE) computed tomography (CT) imaging. The uncertainties of kinetic parameters (e.g. perfusion) may stem from uncertainties in the uptake curve and arterial input function measurement, which is in turn determined by the scanning technique. In addition, there usually is an implicit assumption in CT perfusion studies that the enhancement in Hounsfield unit (HU) measurement in the vasculature and tissues is linearly proportional to their concentration of contrast agent. As long as this relationship is the same for both artery and tumor, the calibration of contrast concentration versus HU typically is not required. Typically, this is rarely the case, especially not in the thoracic region, where beam hardening effects may occur both due to potential contrast in superior vena cava, air/tissue inhomogeneities and differences in patient size [16]. In addition, there is an increasing use of absolute HU thresholds for functional analysis as well as segmentation of tissues of interest. As such it is now recognized that there is a clear need for not only calibrating the CT system for sensitivity to iodine in both relative and absolute terms but also for assessing the stability of the calibration over time [17]. Despite this recognition, there are currently no perfusion- or dynamic contrast calibration phantoms commercially available or discussed in the literature.
Perfusion imaging provides the ability to detect regional and global alterations in organ blood flow [1]. In tumors, an increase in perfusion is typically associated with the process of angiogenesis and this has proven to be related to the staging of disease [2] and treatment response [3].
One possibility to measure tissue perfusion is through the use of dynamic contrast-enhanced (DCE) computed tomography (CT) imaging. With DCE-CT, the redistribution of a contrast agent after a bolus injection can be visualized as a time-sequence. DCE-CT has been extensively developed during the last decade, not least because of enormous progress in computed tomography scanner technology. It may be an attractive imaging modality because of its simplicity of quantification (linearity between injected contrast concentration and CT contrast enhancement) and the possibility to add it to conventional anatomical CT examinations [2].
One potential application of DCE-CT is the measurement of liver and lung perfusion in order to distinguish malignancy from normal tissue and/or fibrosis [4]. However, in order to apply DCE-CT to these or other organs that are subject to breathing motion, motion artifacts should be considered.
Not only may the contrast in the moving organ be subject to reconstruction artifacts, it may also be affected by motion artifacts, such as motion-induced blurring. This may be a ubiquitous and noticeable artifact when imaging respiratory-sensitive organs in free-breathing conditions [5].
For dynamic imaging applications, motion-induced blurring may add temporal variations to the measured contrast enhancement values, which may lead to errors in the quantification and therefore to artifacts of the resulting parametric images. In addition, if the imaging field-of-view is limited, the vessel(s) of interest may periodically move out of the plane of interest resulting in further loss of contrast enhancement sampling.
Reducing these artifacts is not straightforward. DCE-CT imaging of the liver is usually performed under breath hold [4] [6]. In principle, reconstruction artifacts and motion-induced blurring may be minimized with a high-temporal resolution, for example by employing a high scanning frequency [7]. This is becoming feasible with current state-of-the-art multi-detector CT systems that enable a variety of noninvasive techniques with unprecedented spatial and temporal resolution [8]. Often, however, multi-detector scanners are still limited in the field-of-view in cradio-caudal extent which limits the ability to control out-of-plane respiratory motion [9]. The latest developments in CT scanner technology (e.g., Aquilion One, Toshiba, Tochigi Pref., Japan) may offer advantages over conventional CT systems through the capability for volumetric scanning of high speed and increased coverage within a single gantry rotation. Such a 320-slice CT scanner has been used for implementation into routine CT simulation, offering sub-millimeter spatial resolution at cranio-caudal coverage of 160 mm in a single rotation of 0.35 [10]. This volumetric scan mode may be capable of mitigating both the lack of temporal scan resolution as well as limited field-of-view.
A potential drawback of faster acquisition with a large-field-of-view may be a decreased image quality. Therefore, a scanning protocol that includes considerations about image quality and, at the same time, patient dose, may not always employ the fastest gantry rotation time.
Conventional calibration phantoms typically are designed for use only in static conditions, and hence may not be useful when considering motion artifacts. Other phantoms may be designed mainly for quantifying volume changes under motion, rather than for contrast calibration.